How Does Mass Migration Benefit the Poor?

Genuine question to anyone that supports, or advocates for mass migration: How does it benefit the poor?

I can't see any mechanism whereby the mass immigration of immigrants benefits the poor of our society.

How does it benefit the 16 year old starting to get on the job ladder?

How does it benefit the 20-something wanting to buy a house?

How does it benefit the middle-aged poor person?

Where, in the great scheme of things does mass migration benefit the poorest in society? 

It's a genuine question I'd love someone to answer. 

Show me where mass immigration of unskilled people is a benefit, a help, a positive to the working poor of this country.

Because I'd like someone to explain why the working poor of this country don't have a legitimate grievance against mass immigration.